The "not" that is implicit in "unless" is subtle enough that it can get "lost" when you are looking at code.

I never had that problem. For me, the "not" cannot be lost, because without it it's not unless any more.

This shows that not only shouldn't you use "unless" even for simple conditionals, but shouldn't even consider using "unless".

No, it does not. That's an undue generalization. You may not advocate the banishment of a language construct as a general rule just because "quite a few" got "burned by it now". There's no count of those which never got burned by using 'unless', and which are quite comfortable using it as a statement modifier or a control construct in the sense perlsyn states:

"if" executes the statement once if and only if the condition is true. "unless" is the opposite, it executes the statement unless the condition is true (i.e., if the condition is false).

Subtleties in the english language regarding 'unless' are irrelevant and must not be stressed to daemonize it's use. After all, we're programming perl, not writing english statements.